1. Field of the Invention
The present invention relates to a communication apparatus and a communication method, and more particularly a communication apparatus and a communication method applicable to, for example, a telecommunications network system, such as a sensor network, including many wireless communication nodes distributed in an area to achieve mutual data communication between the nodes on wireless signals.
2. Description of the Background Art
Specifically in communication nodes forming a telecommunications network, such as a sensor network, the saving of power consumption is demanded. En-Yi A. Lin, et al., “Power-Efficient Rendez-vous Schemes for Dense Wireless Sensor Networks”, Proceedings of ICC (IEEE International Conference on Communications) 2004, Paris, France, June 2004, discloses a solution in which a wireless device performs an intermittent startup operation.
According to the communication technique taught by En-Yi A. Lin, et al., the wireless device is powered on in a short period of time at a predetermined time interval for waiting for signal reception and powered off in the remaining period of time, thereby saving the power consumption. This periodic timing for waiting is called “sniff timing”.
When a transmitter node intends to transmit a data packet to a receiver node, the transmitter node is required to transmit the packet in time with the sniff timing of the receiver node. However, it is generally difficult to know the sniff timing of other nodes such as the receiver node.
In the prior art solution, if traffic arises, the transmitter node successively sends startup request packets. This makes it possible to deliver a startup request packet in course of time to the receiver node, or destination, within its sniff timing. In response, the receiver node transmits a startup response packet to the transmitter node, or source. The source node is thus notified of the destination node now being in its reception waiting state, and then transmits data packets in correct timing.
The a communication technique taught by En-Yi A. Lin, et al., is called “power-saving listening scheme”, and is implemented in a remarkably simple manner and may accomplish excellent power-saving effect.
The power-saving of the intermittent operation of communication technique becomes more effective by setting the period of time assigned for signal reception waiting in the sniff timing as short as possible. Instead, if the time for signal reception waiting is made too short, it is then possible for a destination node to receive only part of the startup request transmitted from another node, thus failing to correctly receive the startup request packet. In this case, the startup request packet has been received not entirely but partially, thus exhibiting the high level in received signal strength.
The nature of the high received signal strength can be utilized for solving the above-mentioned problem. That is, the default value of the time for signal reception waiting is set shorter, and carrier sensing is continuously carried out during such a shorter waiting time. If the high level in received signal strength is continuously detected in the carrier sensing, then the signal reception waiting time is somewhat extended so as to entirely receive the startup request packet.
The manner stated above may utilize a reference or threshold level to determine whether the received signal strength is in the high or the low level. Such a reference level is called a “carrier sense level”. The carrier sense level is a parameter also relating to the collision avoidance system, such as CSMA (Carrier Sense Multiple Access) system, for use in packet transmission. The value of the carrier sense level is generally selected in consideration that the packet transmission does not conflict with other systems.
The value of the carrier sense level is therefore selected suitably for preventing the collision by the CSMA system or the like. There is however another problem that the value is not necessarily suitable to determine in the power-saving listening scheme whether or not the startup request packet is being transmitted at the sniff timing from a neighboring node.
Furthermore, the suitable value of carrier sense level for determining whether or not a startup request packet is being transmitted from a neighboring node depends upon how the nodes are distributed and how radio waves propagates. Therefore, if the suitable value of the carrier sense level is selected in advance when designing a telecommunications system, it may be costly.